Recently, the degree of integration of semiconductor memories or semiconductor devices has been rapidly progressing. For example, in a dynamic random access memory (hereinafter, the memory will be referred to as a DRAM), the degree of integration has developed at such a rapid pace that the number of bits of the DRAM has increased fourfold in three years. This development achieves such objects as increasing processing speed of the device, reducing power consumption of the device, and reducing device cost. However, no matter how highly the degree of integration is developed, a capacitor, which is a component of the DRAM, must have a certain capacitance. In consequence, it is necessary make a capacitor material film thinner, but there is a limitation when SiO.sub.2 or Si.sub.3 N.sub.4, which is conventionally used as a capacitor material, is used. However, when a suitable capacitor material having a high dielectric constant is used instead of SiO.sub.2 or Si.sub.3 N.sub.4, it is possible to increase the capacitance without changing the film thickness. Therefore, recently, research into utilizing a capacitor material having a high dielectric constant for the memory device has been given attention.
Of the characteristics required for the capacitor material, the most important ones are high dielectric constant and small leakage current, even when the film is thin. In other words, it is necessary to use a capacitor material having a high dielectric constant, as well as to make the film as thin as possible and to minimize leakage current. In general, a rough objective of the development may be considered that the film is no more than 0.6 nm thick in SiO.sub.2 equivalent thickness and the leakage current density upon the application of 1.1 V volts is less than or equal to a current on the order of 10.sup.-8 A/cm.sup.2. Further, when the thin film is formed on an electrode for a capacitor of a DRAM having step-shaped portions, it is very advantageous that the thin film be formed by CVD which has good step coverage on a complicated topography. From this point of view, there have been examined procedures of forming the thin film by means of various film forming techniques using tantalum oxide, lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), strontium titanate, or barium titanate. However, although the formation of the thin film by CVD is the most advantageous, there is a problem that there exists no CVD source material which has a stable and good vaporization property. There is a problem because the vaporization property of dipivaloylmethanato (DPM) compounds, .beta.-diketones, which are widely used as a main CVD source material, by heating is not good. Thus, according to the described difficulties, a technique for producing a dielectric thin film which has good performance and good repeatability has not yet been established.
In such a situation, some of the inventors of the present invention formerly proposed, in the specification of the Japanese Patent Application No. 4-252836, a CVD source material having an extremely high vaporization property which comprises a solution in which a conventionally used solid material is dissolved in an organic solvent, such as tetrahydrofuran (THF). However, the inventors of the present invention attempted to produce a dielectric thin film using this CVD source material in a conventional thin film depositing apparatus (for liquid material) for producing an SiO.sub.2 film etc. by CVD, and found that there were various problems in the apparatus and the process for producing the thin film.
Hereinafter, there will be described a conventional thin film depositing apparatus or method for forming a barium strontium titanate (BST) thin film for capacitor use by CVD using the described CVD source material. As the CVD source material of a Ba or Sr group, a solution in which a compound of Ba or Sr with dipivaloylmethanato (DPM), which is an organometallic complex, is dissolved in tetrahydrofuran (THF) at a concentration of 0.01 to 1 mol/liter, is used. Further, as the CVD source of Ti, titanium tetraisopropoxide (TTIP) is used.
FIG. 12 is a schematic view illustrating a structure of a conventionally known CVD thin film depositing apparatus using liquid source materials.
As shown in FIG. 12, the thin film depositing apparatus (CVD apparatus) comprises a carrier gas supply tube 1 for supplying a carrier gas, such as argon, a carrier gas flow controller 2, such as a mass flow controller, and a connecting tube 3 with a downward end connected to a vaporizer 4. Further, the thin film depositing apparatus includes a plurality of liquid material containers 5, each of which contains a liquid CVD source material, a plurality of liquid material feeders 6 (liquid material flow controllers), such as measuring pumps etc., a plurality of pressure tubes 21 for feeding the liquid CVD source materials by pressure, and a nozzle 7 for atomizing the liquid CVD materials. Each of the liquid material containers 5 is made of 314 stainless steel which has a high corrosion resistance property. Still further, the thin film depositing apparatus has a heater 8 for heating the vaporizer 4, the heater 8 being designed to heat the vaporizer 4 and to hold the temperature of the vaporizer 4 at a predetermined constant value in accordance with an output of a temperature detecting sensor (not shown). In order to make temperature distribution uniform within the wall of the vaporizer 4, the wall of the vaporizer 4 is made of metal having high thermal conductivity, such as aluminum. The downward end of the nozzle 7 is located in a throttle portion 9 of the connecting tube 3 in which the flow rate of the carrier gas is maximum and is cut diagonally to atomize the supplied CVD source material efficiently.
Moreover, the thin film depositing apparatus comprises material gas transport piping 10, a heater 11 for keeping the material gas transport piping 10 warm, a material gas supply hole 12, and a reactive gas supply tube 13. The reactive gas supply tube 13 supplies an oxidant. Further, the apparatus includes a heater 14 for a reaction chamber 15, the heater 14 being designed to heat the wall of the reaction chamber 15 to prevent recondensation of the CVD source material or powder adhesion. The apparatus also has substrate heating equipment 16 (heating stage) for heating a substrate 17 made of silicon on which a thin film is to be formed. Moreover, the apparatus includes a load-unload chamber 19 connected to the reaction chamber 15 through a gate 20 (gate valve) and a handler 18 (substrate handler) for moving the substrate 17 between the reaction chamber 15 and the load-unload chamber 19.
A procedure of depositing the thin film by CVD using the described CVD materials and the thin film depositing apparatus is as follows (CVD process). At first, each of the vaporizer 4, material gas transport piping 10, and the reaction chamber 15 is heated by the corresponding heaters 8, 11, 14, or 16 to reach a predetermined temperature, respectively, and then the substrate 17 is transferred from the load-unload chamber 19 onto the substrate heating equipment 16 through the gate 20 by the handler 18. When the substrate 17 is heated and reaches a predetermined temperature, a carrier gas bomb (not shown) is opened so that carrier gas for dilution is introduced into the vaporizer 4 through the carrier gas supply tube 1, the carrier gas flow controller 2, and the throttle portion 9 of the connecting tube 3.
Then, each of solutions in which bis (dipivaloylmethanato) barium is dissolved in THF, a solution in which bis (dipivaloylmethanato) strontium is dissolved in THF, and a solution in which titanyl bis (dipivaloylmethanato) is dissolved in THF is fed as a liquid CVD source material at a constant flow rate from a corresponding one of the liquid material containers 5 to the vaporizer side. Each of the CVD materials is atomized at the end portion of the nozzle 7 by the surrounding carrier gas flowing at high speed, and then the atomized CVD material particles collide over a wide range of the inner wall of the vaporizer 4 and are vaporized instantaneously. Since the carrier gas flows at high speed around the surface of each of the liquid particles which is vaporizing, the vaporization and mixing of the CVD material is improved (scavenging effect). Further, mixing between the CVD material gas, which is vaporized in the material gas transport piping 10, and the carrier gas is improved further so that the BST film is formed (deposited) on the surface of the substrate 17 by the gas mixture introduced into the reaction chamber 15 through the material gas supply hole 12 according to the CVD reaction.
By precisely observing the surface of the substrate 17, the BST film is deposited on the upper and side surfaces of a storage node and on the surface of an interlayer insulating film. After the supply of each of the CVD source materials is stopped, the substrate 17 is transferred to the load-unload chamber 19 by the handler 18 again and the thin film forming process is completed.
Moreover, when a strontium titanate (SrTiO.sub.3) thin film is formed by using a solution of Sr(DPM).sub.2 in THF and a solution of TiO(DPM).sub.2 in THF or TTIP as CVD source materials, and also using O.sub.2 as an oxidant, the thin film is also formed by using the thin film depositing apparatus shown in FIG. 12. It is probable that TTIP, which is liquid at normal temperature and has high vapor pressure, is fed by bubbling.
In this case, after the vaporizer 4 has been heated by the heater 8 to reach a predetermined temperature, up to 250.degree. C., an inert carrier gas for dilution with a constant flow rate controlled by the carrier gas flow controller 2 is injected into the vaporizer 4 through the nozzle 7. Each of the solutions of Sr(DPM).sub.2 in THF and TiO(DPM).sub.2 in THF, each of which is a liquid CVD source material, is fed to the vaporizer at a constant flow rate. Each of the CVD source materials is atomized at the end portion of the nozzle 7 by the surrounding carrier gas flowing at high speed, and the atomized CVD material particles collide with a wide range of the inner wall of the vaporizer 4 and are vaporized instantaneously. Each of the CVD materials which has been vaporized is transferred to the reaction chamber 15 through the material gas transport piping 10 and mixed with an oxidant gas (for example, O.sub.2 or N.sub.2 O). The gas mixture is introduced into the reaction chamber 15 maintained at constant pressure so that the SrTiO.sub.3 film is formed on the surface of the substrate 17 heated by the substrate heating equipment 16 according to the CVD reaction. When the SrTiO.sub.3 film is formed using TTIP instead of TiO(DPM).sub.2, the TTIP vaporized by bubbling is fed through the reactive gas supply tube 13. The remainder of the gas mixture, which does not contribute to the thin film formation, is discharged through an exhaust line by a vacuum pump.
However, in the described conventional thin film depositing techniques, there are problems as follows.
(1) In forming the thin film by means of the described thin film depositing system using CVD, since the mutual ratio among Ba, Sr, and Ti at the early stage of film forming is determined by the vaporization property of each of the CVD source materials, it is impossible to precisely control the content of each of Ba, Sr, and Ti in the early stage of film forming, the content being an important factor for improving the crystallinity of the thin film, which controls the electrical properties of the BST film.
(2) In forming the thin film by means of the described thin film depositing system using CVD, the quality of the BST crystal at the early stage of film forming controls the electrical properties of the whole BST film. Any heat treatment for improving crystallinity takes a relatively long time for raising the film temperature, and the throughput is reduced.
(3) The CVD process has a substrate selectivity due to decomposition of the materials. From a microscopic view, it is observed that an abnormal morphology is caused on the surface of the interlayer insulating film. Further, since it is probable that the content of each of Ba, Sr, and Ti in the BST film formed on the side wall of the storage node, near the surface of the interlayer insulating film on the substrate, is out of order, the electrical properties of the thin film may be inferior.
(4) On the storage node, it is required that the film have a high dielectric constant and low leakage current density in order to act as a capacitor insulating film, while on the surface of the interlayer insulating film, it is required that the film have a low dielectric constant in order to elevate its independence between adjacent storage nodes. When the BST film is somewhat thick, the BST film on the surface of the interlayer insulating film is also crystallized so that the dielectric constant is increased.
(5) In order to give the SrTiO.sub.3 film any desired electrical properties, an extremely exact content control with an error within .+-.0.3% in the ratio of (number of Sr atoms)/(number of Ti atoms) is required. However, since the supply flow rate (quantity of supply per unit time) of each of the liquid CVD source materials is very small, it is difficult to exactly control such a very small flow rate by means of the liquid material feeder. Further, since the response of the liquid material feeder is poor, there occurs an over-shoot in the flow rate when the valve is opened at the start of the film forming step and the quantity of the material gas formed by vaporization increases in response to the over-shoot. In consequence, it is difficult to form a thin film having constant composition with good accuracy.
(6) In order to improve the vaporization efficiency of Sr(DPM).sub.2, the space in the vaporizer is held at a high temperature (for example, about 250.degree. C.) and low pressure (for example, about 30 Torr). The inner space of each of the nozzle and the piping connected to the nozzle is also under a low pressure since each of the inner spaces communicates with the vaporizer. Further, it is impossible to independently control the pressure of each of the inner spaces since the pressure changes in response to the pressure in the vaporizer. In such a condition, a solvent having high vapor pressure (for example, THF) tends to vaporize very easily, while Sr(DPM).sub.2 having low vapor pressure tends to remain in a liquid state. Thus, material concentration of the liquid in the pipes increases so that the viscosity of the liquid increases. In consequence, the flow rate of each of the CVD source materials is reduced, and it is probable that a constriction is formed in the piping.
(7) When each of the CVD source materials is fed from a corresponding one of the liquid material containers to the vaporizer, respectively, each of the CVD source materials is not fed uniformly so that the thickness of the thin film cannot be uniform.
(8) It has been found that in forming a thin film of SrTiO.sub.3, the quality of the thin film is more influenced by the kind of Ti material than the kind of Sr material. When TTIP, with a probability of sticking to the substrate, .beta., (.beta.-0.5 at a substrate temperature Ts=420.degree. C. and a reactor pressure P=1.5 Torr) is used, a thin film of SrTiO.sub.3 having a plane and a uniform structure is formed regardless of the quality of the substrate (under layer). However, when the thin film is formed on the substrate for a super LSI having a three-dimensional structure with a height up to 1 micron, if the film forming temperature is raised to or above 400.degree. C., the temperature required for crystallization, a film forming reaction progresses at the upper surface of the step-shaped portion at which the CVD material first arrives, so that the CVD material is consumed. In consequence, the thin film is thinner at the side of the step-shaped portion. Namely, there occurs a fault such that the covering property of the thin film (step coverage) is reduced. When TiO(DPM).sub.2 with a .beta. (.beta.-0.1 at Ts=420.degree. C. and P=1.5 Torr) is used, the covering property of the thin film is good. However, there occurs a fault such that the deposition property, particularly the deposition property on the under-layer of silicon oxide, is reduced according to the material species of the under-layer. The fault becomes remarkable when the film forming temperature is low. When the temperature is no higher than 450.degree. C., the density (probability) of generation of initial cores by CVD reaction is low so that the thin film does not grow uniformly on the substrate. In consequence, there is formed a thin film in which the size of each of particles is diverse or the grain boundary of each of the particles is not clear, and also the structure of the film is not uniform.
(9) In order to supply the CVD source material gas formed by vaporization in the vaporizer, whose temperature is maintained at 250.degree. C., to the reaction chamber without causing condensation or decomposition of the gas, it is necessary to hold the temperature of the material gas transport piping between the vaporizer and the reaction chamber at 250.degree. C. Therefore, the material gas transport piping, which comprises tubes of SUS and a valve, is heated and held at 250.degree. C. by the heater (ribbon heater). However, since the heat loss from the heater to the atmosphere is significant, the heat efficiency of the heater is poor and the heat uniformity at each portion of the piping is poor.
(10) In forming the BST thin film by CVD using liquid CVD source materials, it is difficult to accomplish good repeatability of composition and film quality due to deterioration of the materials, because an adequate system for monitoring the process in situ does not exist.
(11) The susceptor for supporting the substrate in the reaction chamber is conventionally made of carbon. When formation of the thin film on the substrate is repeated about sixty times, the susceptor is deteriorated due to oxidation of carbon so that the temperature distribution within the substrate becomes non-uniform. Further, as the formation of the thin film is repeated, the temperature of the substrate changes.
(12) The temperature of the gas nozzle is determined according to the amount of heat radiated from the heating stage and the amount of heat discharged into cooling water 25 flowing above a gas head. However, it is impossible to control the radiated or discharged heat freely.
(13) The CVD material gas formed by vaporization in the vaporizer is mixed with reactive gas, further passes through the gas nozzle, and then reacts on the substrate to form the thin film. A part of the product produced by the reaction, which has not contributed to the film formation, adheres to the inner wall of the reaction chamber so that it is probable that particles will adhere to the substrate.
(14) Since the temperature of the bottom side of the reaction chamber is comparatively low, a part of the product produced by the reaction, which has not contributed to the film formation, tends to condense and adhere on the bottom side. Thus, when the reaction chamber is evacuated by a vacuum pump or restored to atmospheric pressure, the particles deposited on the bottom side are blown upward so that it is probable that the particles will adhere to the substrate.
(15) Since the composition of the CVD material gas, which is formed by vaporization in the vaporizer and introduced into the reaction chamber, is unstable, the composition of the thin film formed on the substrate is also unstable.
(16) Since the liquid CVD source material collides with the inner wall of the vaporizer to be vaporized, the residue produced adheres to the inner wall of the vaporizer and is scattered in the vaporizer and is introduced into the reaction chamber together with the source material gas.
(17) When the inner wall of the vaporizer is contaminated due to adhesion of the residue from the material, it is troublesome to clean the inner wall of the vaporizer.